Anisotropic magnets produced by milling magnetocrystalline anisotropy materials such as ferrites or rare-earth alloys and pressing the milled magnetic material in a specific magnetic field are widely used in speakers, motors, measuring instruments and other electrical devices. Of these, in particular, magnets with anisotropy in a radial direction are endowed with excellent magnetic properties, are freely magnetizable and require no reinforcement to fix the magnet in place as in the case of segment magnets, finding use in AC servomotors, DC brushless motors and other related applications. The trend in recent years toward higher motor performance has brought with it a demand for elongated radially anisotropic magnets.
Magnets having a radial orientation are manufactured by vertical compacting in a vertical magnetic field or by backward extrusion. The vertical compacting in vertical magnetic field process is characterized by applying opposing magnetic fields through the core of a mold in the pressing direction so as to provide a radial orientation. That is, as shown in FIG. 2, a magnet powder 8 packed into a mold cavity is radially oriented by coil 2 to generate orienting magnetic fields which are opposed toward each other through cores 4 and 5, to thereby form magnetic circuits that run from the cores 4 and 5 to a die 3 and back to the cores through a compactor frame 1. Also shown in FIG. 2 are a top punch 6 and a bottom punch 7.
Thus, in this vertical compacting in vertical magnetic field apparatus, the magnetic fields generated by the coils create magnetic paths extending from the cores, through the die and the compactor frame and back to the cores. To reduce magnetic field leakage loss, a ferromagnet, typically a ferrous metal is used as the material making up the portions of the compactor that formed the magnetic paths. However, the strength of the magnet powder-orienting magnetic field is determined by the following parameters.
Magnetic fluxes which have passed through the top and bottom cores meet from opposite directions at the core center and divert into the die. The amount of magnetic flux that passes through the core is determined by the saturation flux density of the core. The magnetic flux density of an iron core is about 2.0 T. Therefore, the strength of the orienting magnetic field at inside and outside diameters of a magnet powder packed cavity is obtained by dividing the magnetic flux which has passed through the top and bottom cores by the inside surface area and outside surface area of the magnet powder packed cavity, respectively, as follows:2·π·(B/2)2·20/(π·B·L)=10·B/L (inner periphery);2·π·(B/2)2·20/(π·A·L)=10·B2/(A·L) (outer periphery)wherein B is a core diameter (magnet powder packed cavity inside diameter), A is a die diameter (magnet powder packed cavity outside diameter), and L is a magnet powder packed cavity height. Because the magnetic field is smaller at the outer periphery than at the inner periphery, a magnetic field of at least 10 kOe is required at the outer periphery in order to obtain good orientation in all areas of the magnet powder packed cavity. As a result, 10·B2/(A·L)=10, and so L=B2/A. Given that the height of the green compact is about one-half the height of the packed powder and is reduced further during sintering to about 80%, the magnet ultimately obtained has a very small height. Because the saturation flux density of the core determines the strength of the orienting magnetic field as mentioned above, the size (i.e., height) of the magnet that can be oriented is dependent on the core shape. It has thus been difficult to manufacture annular magnets that are elongated in their axial direction. In particular, it has been possible to manufacture small-diameter annular magnets only to very short lengths.
The backward extrusion process for manufacturing radially oriented magnets is not effective to the production of low-cost magnets because it requires the use of large equipment and has a poor yield.
Thus, regardless of which method is used, radially anisotropic magnets are difficult to manufacture. The inability to achieve the low-cost, large-volume production of such magnets has in turn made motors that use radially anisotropic magnets very expensive to manufacture.
In the event radially anisotropic ring magnets are produced as sintered magnets, as a result of anisotropy imparted, unwanted fracture and cracking can occur in the magnet during the sintering and aging/cooling steps, if the stress generated in the magnet due to a difference in coefficient of linear expansion between a c-axis direction and a direction perpendicular to c-axis is greater than the mechanical strength of the magnet. For this reason, R—Fe—B base sintered magnets could be manufactured only to a magnet shape having an inner/outer diameter ratio of at least 0.6 (see Hitachi Metals Technical Report, Vol. 6, pp. 33-36). Further, in the case of R—(Fe,Co)—B base sintered magnets, cobalt that has substituted for iron is not only contained in the primary 2-14-1 phase in the alloy structure, but also forms R3Co in an R-rich phase, reducing mechanical strength noticeably. Due to a high Curie temperature, in addition, there occur greater changes in coefficient of thermal expansion in a c-axis direction and a direction perpendicular to c-axis during the cooling step from the Curie temperature to room temperature, resulting in an increased residual stress which causes fracture and cracking. For this reason, R—(Fe,Co)—B base radially anisotropic ring magnets are given still stricter shape limits than cobalt-free R—Fe—B base magnets, so that stable magnet manufacture is possible only with a shape having an inner/outer diameter ratio of at least 0.9. The problem becomes serious particularly in the case of small-diameter radial magnets since they have a low inner/outer diameter ratio despite a thickness. For the same reason, ferrite magnets and Sm—Co base magnets suffer fracture and cracking, prohibiting their stable manufacture.
The circumferential residual stress, associated with radial anisotropy imparted, which causes fracture or cracking to occur during the sintering and aging/cooling steps is discussed in the report of Kools' study relating to ferrite magnets (F. Kools, Science of Ceramics, Vol. 7 (1973), pp. 29-45) and expressed by equation (1).σθ=ΔTΔαEK2/(1−K2)·(KβKηK−1−Kβ−Kη−K−1−1)  (1)
σθ: circumferential stress
ΔT: temperature difference
Δα: difference in coefficient of linear expansion(α∥−α⊥)
E: Young's modulus in orienting direction
K2: anisotropic ratio of Young's modulus (E⊥/E∥)
η: position (r/outer diameter)
βk: (1−ρ1+K)/(1−ρ2K)
ρ: inner diameter/outer diameter ratio
In the above equation, the item having the largest impact on the cause of fracture or cracking is Δα, i.e., difference in coefficient of linear expansion (α∥−α⊥). For ferrite magnets, Sm—Co base rare earth magnets and Nd—Fe—B base rare earth magnets, the difference in coefficient of thermal expansion between different crystal directions (i.e., anisotropy of thermal expansion) develops from the Curie temperature and increases as the temperature lowers during the cooling step. At this stage, the residual stress increases beyond the mechanical strength of magnet, resulting in fracture.
The stress due to the difference in thermal expansion between the orientation direction and a direction perpendicular thereto, as given by the above equation, develops as an annular magnet is radially oriented over its entire circumference. Therefore, if an annular magnet including a portion which is oriented differently from the radial orientation is produced, the occurrence of fracture is suppressed. For example, annular magnets which are prepared by the vertical compacting in horizontal magnetic field process so that they are oriented in one direction perpendicular to the annular axis do not fracture regardless of whether they are Sm—Co base rare earth magnets or Nd—Fe(Co)—B base rare earth magnets.
Fracture occurs just because of radial orientation. In a method generally taken for radial magnets for preventing fracture, the radial orientation of a radial magnet is disordered so as to reduce the difference between thermal expansion in c-axis direction and thermal expansion in a direction perpendicular thereto. This method, however, reduces the magnetic flux from the magnet serving as a torque source for a motor, failing to construct high-performance motors.